1. Field of the Invention
Embodiments of the present invention generally relate to an apparatus and a method for processing semiconductor substrates. Particularly, embodiments of the present invention relates to methods and apparatus for performing a bottom up plating.
2. Description of the Related Art
With continuous decrease of semiconductor device dimensions, trench or via structures formed on a semiconductor substrate during fabrication become increasingly narrow and high in aspect ratio. Narrow opening and high aspect ratio usually present difficulties and challenges for subsequent material filling process. As a result, voids are more likely to form in the filling material because the narrow openings would be pinched off during filling process.
The problem becomes more pronounced when a liner, barrier, or seed layer is requested for the filling process, such as in a plating process where a seed layer is required. The seed layer is usually deposited on the trench or via structure using a physical vapor deposition (PVD) process. To achieve a complete coverage over the trench or via surfaces, the seed layer is usually thicker near an entrance of the trench or via and near a bottom of the trench or via. The thick portion of the seed layer has smaller electrical resistance, therefore attracting more plating current and resulting in quick plating. As a result, plating is faster near the opening of the trench or via and material plated near the opening pinches off the opening before the trench or via is filled, forming voids in the trench or via. The voids are not desirable because they reduce conductivity of the interconnect formed in the trench or via and also reduce physical strength of the interconnect.
FIG. 1 schematically illustrates problems in plating trench and via. Trench or via structures 14 are formed in a dielectric material 11 of a substrate 10. A barrier layer 16 is then deposited over the trench or via structures 14. A seed layer 12 is then deposited over the barrier layer 16. The seed layer 16 is thicker near entrances 17 of the trench or via structures 14 resulting in faster plating near the entrances 17. A metal layer 13 is then deposited to fill the trench or via structures 14. As shown in FIG. 1, voids 15 are formed in trench or via structures 14 with high aspect ratio during deposition of the metal layer 13.
Conventional fabrication process generally uses a sputtering process to reduce the thickness of the seed layer 12 near the entrances 17 prior to plating. Sputtering generally uses positive ions to physically knock out atoms in the seed layer. The positive ions, such as positive argon ions, are usually generated in a plasma chamber and then accelerated towards the target. The positive ions gain momentum during acceleration and strike top surfaces of the substrate. The ions physically knock out atoms in the seed layer. However, the ions strike the entire substrate. Additionally, the dislodged particles generated during sputtering require additional cleaning process and may still become potential source of contamination for subsequent processing.
Therefore, there is a need for methods and apparatus for improving plating conductive materials in trench or via structures.